Red Stains on Concrete — Corrosion of Rebar
David W. Harris, PhD,PE, F.SEI, F.ASCE and Mallory A. Westbrook
Introduction
Corrosion in concrete is a major problem affecting all concrete structures. There are approximately 350,000 concrete bridges in the United States with approximately 15% of these bridges being listed as structurally deficient due to corrosion of metal components. In 2002, the Federal Highway Administration estimated that the annual cost of corrosion for highway and bridges alone was $8.3 billion annually (Koch, et al, 2002).
These large costs do not account for the corrosion ongoing around us everywhere in driveways, pavements, retaining walls, and concrete structures of all kinds.
This article explores the causes of concrete corrosion and possible techniques to prevent future damage due to corrosion.
What causes concrete corrosion?
ASTM defines corrosion as “the chemical or electrochemical reaction between a material,usually a metal, and its environment that produces a deterioration of the material and its properties.” For steel embedded in concrete, corrosion results in the formation of rust which has two to four times the volume of the
original steel. Corrosion also produces pits or holes in the surface of reinforcing steel, reducing strength capacity as a result of the reduced cross-sectional area.
Mechanism of Corrosion
In the life of reinforced concrete nature’s balance acts on the reinforcing steel within a concrete structure. Corrosion is an electrochemical process involving the flow of charges (electrons and ions) (PCA, 2002). Steel is more electrically active containing positive iron ions, Fe++, creating an anode. Concrete contains negative charges in hydroxides OH-, creating a cathode (Belkowitz, 2018). The system of anode/cathode seeks equilibrium; the positive anode ions flow to the negative cathode ions, combining with water and oxygen to form rust. This creates a decomposition process.
Initially, this process is beneficial creating a passivity layer, or a protective layer over the steel. This thin layer slows the corrosion process. But as negative ions, water and oxygen remain available additional corrosion can occur causing depassivacation (Belkowitz, 2018). The destruction of the passivating layer occurs when the alkalinity of the concrete is reduced or when the chloride concentration in concrete is increased. For steel in concrete, the passive corrosion rate is typically 0.1 μm per year. Without the passive film, the steel would corrode at rates at least 1,000 times higher (ACI 222 2001).
Chloride concentration in concrete is caused by both internal and external effects. Aggregates in concrete can be a source of chlorides as many were present in geologic times existing under the ancient seas, Even when washed the chlorides are not eliminated from the aggregates. External sources of chloride come from marine environments, salt brine pits, fertilizer, road salts, as examples.
The migration of chlorides is influenced by the wall effect (Belkowitz, 2018). This principle is that the InterFacial Zone closer to hard objects contains less paste, more cracks, and higher porosity. Thus, the zone around reinforcing steel exhibits these properties. Further away from the reinforcing steel the opposite occurs with more paste and lower porosity. For concrete without steel this same principle also applies with denser concrete having a lower porosity.
Of note, even non-reinforced concrete seeks a balance. Carbonation occurs within the concrete. Carbonation occurs when carbon dioxide from the air penetrates the concrete and reacts with hydroxides, such as calcium hydroxide, to form carbonates. In the reaction with calcium hydroxide, calcium carbonate is formed. This reaction reduces the pH of the pore solution to as low as 8.5, at which level the passive film on the steel is not stable. Carbonation is generally a slow process. In high-quality concrete, it has been estimated that carbonation will proceed at a rate up to 1.0 mm (0.04 in.) per year. The amount of carbonation is significantly increased in concrete with a high water-to-cement ratio, low cement content, short curing period, low strength, and highly permeable or porous paste. (PCA, 2002).
Types of corrosion
Crevice Corrosion — Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas). Oxygen in the liquid which is deep in the crevice is consumed by reaction with the metal. Oxygen content of liquid at the mouth of the crevice which is exposed to the air is greater, so a local cell develops in which the anode, or area being attacked, is the surface in contact with the oxygen-depleted liquid.
Pitting — Theories of passivity fall into two general categories, one based on adsorption and the other on presence of a thin oxide film. Pitting in the former case arises as detrimental or activator species, such as Cl-, compete with O2 or OH- at specific surface sites. By the oxide film theory, detrimental species become incorporated into the passive film, leading to its local dissolution or to development of conductive paths. Once initiated, pits propagate auto-catalytically according to the generalized reaction, M+n + nH2O + nCl- → M(OH)n + nHCl, resulting in acidification of the active region and corrosion at an accelerated rate (M+n and M are the ionic and metallic forms of the corroding metal).
Damage caused from concrete corrosion
Spalling- is the result of water entering brick, concrete, or natural stone. It forces the surface to peel, pop out, or flake off. It’s also known as flaking, especially in limestone.
Delamination — Concrete damage known as concrete delamination occurs when the top surface of the concrete is densified or sets up prematurely or before the water in the concrete and air have a chance to reach the surface. This layer is usually thin but it will not allow the air and water from the concrete to pass through to the surface. The dense surface of the concrete blocks the upward motion of water which results from the settling of solids within the concrete mixture. As the water meets the densified thin layer, it pools laterally separating the surface from the body of concrete thereby causing damage to the concrete known as concrete delamination.
How to prevent concrete corrosion?
Coatings
Epoxy coated rebar can be utilized to prevent corrosion in reinforced concrete.
For atmospheric, buried, and marine environment corrosion protection, Zn
(TSZ), Al (TSA), and their alloys have proven that they provide long term
corrosion protection and outperform most all other methods.
Anodic (TSZ/TSA) metal coatings applied to steel cathodes (more noble than Zn or Al), are referred to as cathodic or sacrificial protection coating
systems.
These thermal spray coatings provide corrosion protection by excluding the
environment (or electrolyte) and acting as a barrier coating (like paints,
polymers, and epoxies), but unlike typical barrier coatings they also provide sacrificial anodic protection.
Fly Ash
Use a Fly Ash concrete with very low permeability, which will delay the
arrival of carbonation and chlorides at the level of the steel reinforcement.
Fly Ash is a finely divided silica rich powder that, in itself, gives no benefit
when added to a concrete mixture, unless it can react with the calcium
hydroxide formed in the first few days of hydration. Together they form a
calcium silica hydrate (C-S-H) compound that over time effectively reduces
concrete diffusivity to oxygen, carbon dioxide, water and chloride ions. By
reducing ion diffusion, the electrical resistance of the concrete also increases.
Colloidal Silica admixture
Colloidal Silica is a reactive component that seeks higher porosity and combination with anhydrous calcium hydroxide to electrically balance. The combination creates calcium silica hydroxide, a strong and stable chemical form. This reaction is referred to as an electrical chemical extraction. The chemical formation fills pores and electrically rejects chlorides. Thus, two beneficial effects are produced: the formation of stronger constituents which fill porosity of the concrete, and the purging of negative ions. (Note that negative ions occur in multiple components such as chlorides, potassium, magnesium, and sodium.) These benefits are seen in many structures such as bridges, wharf slabs, and tunnels.
Summary
Corrosion is a major problem in concrete structures particularly reinforced structures. Corrosion is a chemical, electrochemical, and physical attack mechanisms. Damage caused by corrosion includes crevices and pitting in steel reinforcement resulting in lower load carrying capacity and spalling of concrete members. By utilizing the concrete technologies described above, concrete corrosion can become a problem of the past.
References
Koch, G.H, M.P.H Brongers, N.G. Thompson, Y.P. Virmani, J.H. Payer , 2002, “Corrosion Costs and Preventive Strategies in the United States”, PUBLICATION NO. FHWA-RD-01–156.
Video of presentation, Dr. Jon Belkowitz, Thoughts on Aquron migration in concrete structures and the effects on concrete service life. June, 2018, Markum industries, New Zealand.
Portland Cement Association, 2002, Types and Causes of Concrete Deterioration, PCA R&D Serial Number 2617, Obtained from the Internet October 28, 2018.
ACI Committee 222, 2001 Protection of Metals in Concrete Against Corrosion, ACI 222R-01, American Concrete Institute, Farmington Hills, Michigan.
